In this reaction, the ratio of talc and kyanite is dependent on aluminium content with more aluminous rocks favoring production of kyanite. This is typically associated with high-pressure, low-temperature minerals such as phengite, garnet, glaucophane within the lower blueschist facies. Such rocks are typically white, friable, and fibrous, and are known as whiteschist.

Talc is a tri-octahedral layered mineral; its structure is similar to that of pyrophyllite, but with magnesium in the octahedral sites of the composite layers.[2]

Notable economic talc occurrences include the Mount Seabrook talc mine, Western Australia, formed upon a polydeformed, layered ultramafic intrusion. The France-based Luzenac Group is the world's largest supplier of mined talc; its largest talc mine at Trimouns near Luzenac in southern France produces 400,000 tonnes of talc per year, representing 8% of world production.[5]

The structure of talc is composed of Si2O5 sheets with magnesium sandwiched between sheets in octahedral sites.

Talc is used in many industries such as paper making, plastic, paint and coatings, rubber, food, electric cable, pharmaceuticals, cosmetics, ceramics, etc. A coarse grayish-green high-talc rock is soapstone or steatite and has been used for stoves, sinks, electrical switchboards, crayons, soap, etc. It is often used for surfaces of lab counter tops and electrical switchboards because of its resistance to heat, electricity and acids. Talc finds use as a cosmetic (talcum powder), as a lubricant, and as a filler in paper manufacture. Talc is used in baby powder, an astringent powder used for preventing rashes on the area covered by a diaper. It is also often used in basketball to keep a player's hands dry. Most tailor's chalk, or French chalk, is talc, as is the chalk often used for welding or metalworking.

Due to its low shear strength, talc is one of the oldest known solid lubricants. There is also a limited use of talc as friction-reducing additive in lubricating oils.[6]

Talc is widely used in the ceramics industry in both bodies and glazes. In low-fire artware bodies it imparts whiteness and increases thermal expansion to resist crazing. In stonewares, small percentages of talc are used to flux the body and therefore improve strength and vitrification. It is a source of MgO flux in high temperature glazes (to control melting temperature). It is also employed as a matting agent in earthenware glazes and can be used to produce magnesia mattes at high temperatures.

ISO standard for quality (ISO 3262)

Type

Talc content min. wt%

Loss on ignition at 1000 °C, wt %

Solubility in HCl, max. wt %

A

95

4 – 6.5

5

B

90

4–9

10

C

70

4–18

30

D

50

4–27

30

Patents are pending on the use of magnesium silicate as a cement substitute. Its production requirements are less energy-intensive than ordinary Portland cement (at a heating requirement of around 650 °C for talc compared to 1500 °C for limestone to produce Portland cement), while it absorbs far more carbon dioxide as it hardens. This results in a negative carbon footprint overall, as the cement substitute removes 0.6 tonnes of CO2 per tonne used. This contrasts with a carbon footprint of 0.4 tonne per tonne of conventional cement.[7]

Talc is sometimes used as an adulterant to illegal heroin, to expand volume and weight and thereby increase its street value. With intravenous use, it may lead to talcosis, a granulomatous inflammation in the lungs.

Talc powder is a household item, sold globally for use in personal hygiene and cosmetics. Some suspicions have been raised about the possibility its use promotes certain types of diseases, mainly cancers of the ovaries and lungs (it is in the same 2B category in the IARC listing as mobile phones and coffee) although this is not widely recognised as an established link.[8][9]

The studies reference, by subject: pulmonary issues,[10]lung cancer,[11][12] and ovarian cancer.[13] One of these, published in 1993, was a US National Toxicology Program report, which found that cosmetic grade talc containing no asbestos-like fibres was correlated with tumour formation in rats (animal testing) forced to inhale talc for 6 hours a day, five days a week over at least 113 weeks.[11] A 1971 paper found particles of talc embedded in 75% of the ovarian tumors studied.[14] Recent research questions if a link does actually exist between the two. [15][16]

One particular issue with commercial use of talc is its frequent co-location in underground deposits with asbestos ore, which often leads to contamination of powdered talc products with asbestos fibres. Stringent quality control since 1976 (separating cosmetic and food-grade talc from "industrial" grade talc, which is allowed a certain portion of asbestos contamination) has mostly eliminated this issue, but it remains a continuing hazard requiring mitigation in the mining and processing of talc.[18] A 2010 US FDA survey failed to find asbestos in a variety of talc-containing products.[19]